Evaporator and method for producing synthetic fused quartz

ABSTRACT

A method for manufacturing quartz glass, wherein (a) an appropriate liquid starting material is evaporated by spraying it into a vertically arranged evaporation chamber, (b) the vaporous starting material is oxidized to form SiO 2 , and the SiO 2 is collected. The method is characterized in that the starting material to be evaporated is sprayed in on the bottom of the evaporation chamber and the vaporous starting material is removed at the top end of the evaporation chamber, wherein the evaporation chamber is designed such that components depositing in the chamber accumulate on the bottom of the evaporator and are sprayed once again, as well as an evaporator for applying the method.

CROSS REFERENCE TO RELATED APPLICATION

This Utility Patent Application claims the benefit of the filing date ofGerman Application No. DE 10 2013 209 673.9, filed May 24, 2013, andInternational Application No. PCT/EP2014/000660, filed March 12, 2014,both of which are herein incorporated by reference.

BACKGROUND

One embodiment of the invention relates to a method for manufacturingquartz glass as well as to an evaporator adapted to this method.

To manufacture high-purity synthetic quartz glass, use is made ofhalogen-containing and non-halogen silicon compounds as startingmaterials. Halogen-containing feed materials, such as silicontetrachloride (SiCl4) are to disadvantage in that corrosive acids, e.g.,hydrochloric acid (HCl), are produced as side products when they areused. For this reason, use is increasingly made of non-halogen materialswherein, presently, alkyl polysiloxanes are at the center of interest.

According to U.S. Pat. No. 5,043,002, polymethyl cyclosiloxanes such ashexamethyl cyclotrisiloxane (HMCTS), octamethyl cyclotetrasiloxane(OMCTS) and decamethyl cyclopentasiloxane (DMCTS), are particularlyadapted to manufacture high-purity silicon dioxide for optical fibres.The siloxanes are oxidized in a burner flame in the presence of oxygento obtain SiO2 which accumulates in the form of fine amorphous particlesthat are referred to as silica soot. The SiO₂ soot is collected andmelted to form a glass which can be used as starting material forfibre-optic blanks or for optical components.

U.S. Pat. No. 5,356,451 discloses evaporators which are supposed to beadapted to evaporate halogen-containing and non-halogen siliconcompounds. The liquid to be evaporated is applied to an inclined surfacein a thin layer. The surface is heated, with the result that the liquidevaporates while it flows down the surface.

Polymethyl cyclosiloxanes are to disadvantage in that they tend to formresins and gels which result in the contamination of heat exchangersurfaces and burners as well as in the clogging of pipelines. Forexample, resins and gels are formed while the hydrolytic opening of thecyclic alkyl polysiloxanes to form linear hydroxyl-terminated siloxanes(silanols) is in progress, which have a considerably lower volatilitythan the cyclic compounds and deposit in the system. The silanols arereactive and react with cyclosiloxane molecules to form gel-likepolymerisation products. It is also possible that silanol traces arecontained as contaminants in the starting material.

EP 0 719 575 A2 discloses an evaporator for non-halogen siliconcompounds, wherein gels are collected in a sump in order to preventcontamination of evaporator surfaces and clogging of the pipelines. Theevaporator comprises a vertically aligned evaporation chamber into whichthe substances to be evaporated are sprayed. Due to the pressure drop, apart of the pre-heated liquid evaporates when it enters the evaporator,while another part evaporates when it impinges on the heater evaporatorwalls. The gels formed during evaporation are collected in the lowerpart of the evaporator and removed periodically. The nozzles forspraying the liquid are arranged such that they do not carry along thegels having accumulated in the sump of the evaporator.

According to U.S. Pat. No. 6,312,656, the formation of gels is supposedto be prevented in that the alkyl polysiloxanes are not evaporated butare directly sprayed into the burner flame in liquid form. Thetemperature load associated with evaporation and favouring the formationof gels is prevented in this manner.

U.S. Pat. No. 5,879,649 discloses alkyl polysiloxanes having a boilingpoint of less than 250° C., the alkyl polysiloxanes containing less than14 ppm of high boiling contaminants having boiling points in excess of250° C. These contaminants can be evaporated only poorly due to theirhigh boiling points and accumulate in the evaporator where they react toform gel-like deposits. The alkyl polysiloxanes are purified bydistillation and subsequent filtration of the distillate throughactivated charcoal and molecular sieves.

US 2012/0276291 discloses a method for evaporating alkyl polysiloxanesaccording to which the liquid to be evaporated is directed to thevertical walls of an evaporation chamber. The walls of the evaporationchamber are heated to such a high temperature that a part of the liquidevaporates. The remaining liquid flows along the walls and to the bottomof the chamber where it is removed continuously. Gels formed in theevaporator are rinsed out of the chamber along with this liquid.

Despite numerous efforts, the formation of gel is still a severe problemin the manufacture of high-purity quartz glass. The gels can concentratein the evaporator and in the pipelines and impair the process stability.This is particularly applicable for the manufacture of glass fibresbecause, here, minute irregularities considerably impair the furtherprocessing of the glass blank to form fibres. In addition, thepurification of the starting materials and the cleaning of evaporatorsand units are subject to substantial effort.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunder-standing of embodiments and are incorporated in and constitute apart of this specification. The drawings illustrate embodiments andtogether with the description serve to explain principles ofembodiments. Other embodiments and many of the intended advantages ofembodiments will be readily appreciated as they become better understoodby reference to the following detailed description. The elements of thedrawings are not necessarily to scale relative to each other. Likereference numerals designate corresponding similar parts.

FIG. 1 is a sectional view of an evaporator according to one embodiment.

FIG. 2 is a sectional view of an alternative embodiment of an evaporatoraccording to one embodiment.

FIG. 3 is a top view of an intermediate bottom of an evaporatoraccording to one embodiment.

FIG. 4 is a sectional view of the bottom area of an evaporator accordingto one embodiment.

FIG. 5 is a sectional view of an alternatively designed bottom area ofan evaporator according to one embodiment.

DETAILED DESCRIPTION

In the following Detailed Description, reference is made to theaccompanying drawings, which form a part hereof, and in which is shownby way of illustration specific embodiments in which the invention maybe practiced. In this regard, directional terminology, such as “top,”“bottom,” “front,” “back,” “leading,” “trailing,” etc., is used withreference to the orientation of the Figure(s) being described. Becausecomponents of embodiments can be positioned in a number of differentorientations, the directional terminology is used for purposes ofillustration and is in no way limiting. It is to be understood thatother embodiments may be utilized and structural or logical changes maybe made without departing from the scope of the present invention. Thefollowing detailed description, therefore, is not to be taken in alimiting sense, and the scope of the present invention is defined by theappended claims.

It is to be understood that the features of the various exemplaryembodiments described herein may be combined with each other, unlessspecifically noted otherwise.

One embodiment of the invention is based on the object to provide anevaporator and a method for manufacturing quartz glass with which theaforementioned drawbacks can be obviated. In particular, the effortsassociated with the cleaning of evaporators and units as a result of thegel formation are to be minimised as far as possible.

According to the one embodiment of the invention, this object is solvedby a method for manufacturing quartz glass according to which

(a) an appropriate liquid starting material is evaporated by spraying itinto a vertically arranged evaporation chamber of an evaporator,

(b) the vaporous starting material is oxidized to form SiO₂, and

(c) the SiO₂ is collected, wherein

the method is characterized in that the starting material to beevaporated is sprayed in on the bottom of the evaporation chamber andthe vaporous starting material is removed at the top end of theevaporation chamber, wherein the evaporator is designed such thatcomponents depositing in the chamber accumulate on the bottom of theevaporator and are sprayed once again.

The SiO₂ accumulated in step (c) is, preferably, dried in a furthermethod step (d) to form a glass blank and subjected to a heat treatmentfor glazing purposes in step (e). Subsequently, the blank can be furtherprocessed in step (f), e.g. to form glass fibres.

Substances that are possible as liquid starting materials are substancesthat are tending to form gels by polymerisation and adapted tomanufacture SiO₂. Preferred are polymethyl cyclosiloxanes, moreparticularly hexamethyl cyclotrisiloxane (D3), octamethylcyclotetrasiloxane (D4), decamethyl cyclopentasiloxane (D5), ordodecamethyl cyclohexasiloxane (D6). A most preferred starting materialis octamethyl cyclotetrasiloxane (OMCTS; D4). The abbreviations D3, D4,D5, etc. are names the company General Electric introduced forsiloxanes, wherein D stands for the group [(CH₃)₂Si]—O—.

In general, it is preferred to use the particular starting material in aform that is as pure as possible because the evaporation process isadjusted to the boiling point of the starting material. The startingmaterial may contain minor amounts of components having a higher or alower boiling point. For example, the octamethyl cyclotetrasiloxane(OMCTS; D4) that is preferred according to one embodiment of theinvention may contain up to 2 weight per cent of D3 and/or up to 5weight per cent of D5. The content of higher-molecular compounds, suchas D7, D8, D9, etc. is, preferably, not in excess of 30 to 100 ppm.

The temperature in the evaporation chamber is adjusted to the startingmaterial and, in case of OMCTS (boiling point 171° C. to 175° C.; unlessotherwise specified, all data herein relate to normal pressure), ispreferably within a range from 130° C. to 230° C., more preferably from140° C. to 190° C., and most preferably from 150° C. to 180° C.

Preferably, the starting material is sprayed into the evaporationchamber along with a carrier gas. Preferably, the carrier gas used is aninert gas, more particularly nitrogen. It is, however, also possible touse oxygen as carrier gas or to admix oxygen to the carrier gas, theoxygen being required for later oxidation of the alkyl polysiloxanes.

Preferably, the molar ratio of starting material to carrier gas iswithin a range from 0.01 to 2, more preferably from 0.05 to 1, and mostpreferably from 0.1 to 0.75. Among other things, this facilitates theevaporation of higher boiling polymethyl cyclosiloxanes in the startingmaterial, for example of D7 (boiling point 276° C.).

Preferably, the carrier gas has a water content of no more than 30 ppmby volume, more preferably less than 10 ppm by volume.

The vaporous alkyl polysiloxane or, rather, the mixture of carrier gasand vaporous alkyl polysiloxane is removed at the top end of theevaporation chamber and supplied into a burner. Before it is introducedinto the burner, the vaporous material or, rather, the mixture ofvaporous material and carrier gas is, preferably, mixed with oxygen. Inthe burner, the alkyl polysiloxane is oxidized to form SiO₂. Thisresults in the formation of fine amorphous SiO₂ (SiO₂ soot) which isdeposited in the form of a porous mass. In order to ensure completeoxidation of the alkyl polysiloxane, further oxygen is, preferably,supplied in the burner. The supply can be achieved via separate nozzles.

According to an alternative embodiment, the vapour or, rather, themixture of vapour and carrier gas and the oxygen are separately suppliedto the burner and are not mixed before they are in the burner.

Preferably, the burner is additionally charged with a combustion fuel,preferably with methane and more preferably with hydrogen the combustionof which results in an igniting flame into which the vapour or, rather,the mixture of vapour and carrier gas is introduced.

The SiO₂ soot is deposited on a suitable deposition surface, preferablyon a rotating carrier pipe. Herein, a porous SiO₂ body is obtained whichis also referred to as soot body. The soot body is dried in step (d) andconverted into a quartz glass blank by means of a subsequent temperaturetreatment (e). Preferably, drying is achieved at a temperature of 800°C. to 1100° C. Thereafter, the dried soot body is glazed by heating upto a temperature within a range from 1400° C. to 1500° C., i.e. sinteredto obtain a glass blank. Preferably, the porous SiO₂ body is dried in anatmosphere consisting of chlorine gas and an inert gas; preferably,sintering takes place in an inert gas atmosphere, more preferably in avacuum. In either case, helium or nitrogen is preferred as inert gas.Subsequently, the glass blank can be further processed, for example, bydrawing it to obtain glass fibres.

Furthermore, the object of one embodiment of the invention is anevaporator for applying the method, the evaporator comprising avertically aligned evaporation chamber (2) that is limited by walls 2 c,2 d and 2 e, an atomizing nozzle (3) that is arranged on the bottom ofthe evaporator and serves to spray the liquid starting material (4) intothe evaporation space (2 f), and an outlet (8) that is arranged at thetop end of the evaporator, wherein the evaporator is designed such thatcomponents depositing in the chamber accumulate on the bottom of theevaporator and are sprayed once again.

When alkyl polysiloxanes are evaporated, it is practically not possibleto completely suppress the formation of gels and resin-likecontaminants. In order to avoid operational disorders caused by depositsin the unit and to prevent quality from being impaired while the quartzglass is being manufactured, it is, therefore, indispensable to separatesuch contaminants from the vapour flow. According to one embodiment ofthe invention, gels and resin-like contaminants are retained in theevaporator.

FIG. 1 is a sectional view of an evaporator 1 preferred according to oneembodiment of the invention, comprising a cylindrical evaporationchamber 2. The evaporation chamber 2 is limited by a lateral surface 2c, a bottom plate 2 d and a cover 2 e. Within the chamber, there is afree evaporation space 2 f. The evaporator shown in FIG. 1 comprises atwo-substance spray nozzle 3 which is arranged on the bottom of theevaporator. The nozzle 3 has an inner nozzle 3 a and an outer nozzle 3 bwhich surrounds the inner nozzle like a ring. The liquid startingmaterial 3 is supplied to the inner nozzle 3 a and the carrier gas 5 issupplied to the outer nozzle 3 b. The carrier gas and the feed materialare sprayed to form a spray mist 6. The liquid is sprayed into theevaporation space 2 f in a vertical upward direction. In the embodimentshown, the carrier gas 5 and the liquid 4 to be evaporated are sprayedin to the chamber 2 f in the form of a mist, based on the hydraulicpressure principle. At the top end of the evaporator, a mixture 7consisting of the vaporous feed material and the carrier gas is removedthrough the outlet 8.

Non-evaporated contents of the feed material and condensates formed inthe evaporator stay behind when the volatile components are evaporatedand drop to the bottom or they deposit on the chamber walls and flowdown along the latter as long as they are capable of flowing at theevaporator temperature. As a result of gravity, these residuesaccumulate on the bottom of the chamber.

An essential aspect of the method according to one embodiment of theinvention is that the residues which have accumulated on the chamberbottom are sprayed once again. Preferably, they are carried along by theinflowing gas flow and, therein, distributed in the evaporation chamberin the form of fine droplets. Additional nozzles may be provided forspraying the deposits, e.g., nozzles which are charged with carrier gas.As a result of the repeated spraying, vaporisable contents in theresidues, such as condensing starting material, can evaporate in thespray cone and be supplied to the burner. In this manner, the quantityof deposits in the chamber is reduced.

Components the boiling point of which is so high that they do notevaporate under the conditions present in the evaporator once moredeposit and accumulate on the walls and the bottom of the chamber.Usually, these components are high-molecular compounds which are formedby ring opening and polymerisation from cyclic alkyl polysiloxanes.Primarily, the polymerisation is initiated by contaminants which arecontained in the liquid feed material. These are, above all, silanols,water, bases, acids, and chlorides. The presence of such contaminantscannot be avoided from a technical point of view. Initially, thedeposits have a viscous consistency and are capable of flowing under theevaporator conditions. The repeated spraying of these gels favours afurther reaction of the polymerisation products, with the result thatthe molecular weight thereof increases. Herein, the flowability of thecompounds decreases naturally and finally becomes so excessive that,after having deposited on the walls, the deposits cannot flow to thebottom of the chamber any longer. Therefore, the method according to oneembodiment of the invention is characterized in that the flowablecontents are circulated as long as necessary until they assume arubber-like consistency by repeated reaction and become immobile(immobile residues).

As a result of this process, a high-viscosity polymer layer is formed onthe walls and bottoms of the evaporation chamber during ongoingoperation. It has turned out that this layer does not disturb theevaporation process and that the evaporator can be operated withoutinterruption over long time periods despite the formation of this layer.The rubber-like polymers are inert to a large extent. They can remain inthe evaporator and be removed within the scope of maintenance andtesting work that is required anyway. Using an evaporator according toone embodiment of the invention, it was possible to evaporate more than8 tons of OMCTS with an evaporator volume of approx. 10 liters withoutnoteworthy deposits forming in the evaporator. The mean thickness of thepolymer layer was less than 70 μm. The formation of a sump was notobserved.

In contrast to the immobile residues, the residues that are capable offlowing at the evaporator temperatures (mobile residues) still containreactive components and can continue to polymerise. These mobileresidues are continuously reformed while the alkyl polysiloxanes areevaporated.

It has become apparent that the content of resins and gels in the vapourphase removed at the top end of the chamber is astonishingly low. It isassumed that this is to be attributed to the spraying of the mobileresidues. These mobile residues contain linear hydroxyl-terminatedsiloxanes which can react with contaminants. By spraying the mobileresidues in the form of small droplets, the surface thereof is enlargedand the contact with the incoming gas flow is improved. Contaminantspresent in the gas flow are absorbed by the droplets and bound byreaction.

Ultimately, the contaminants are deposited in the evaporator in the formof a high-viscosity immobile polymer layer. Since this layer does notimpair the evaporation process, the evaporator can be operated withoutinterruption over long time periods. To further prolong theuninterruptible operating time, the evaporator can be provided withbuilt-in components which enlarge the inner surface and, therefore, thedeposition surface of immobile deposits. Preferably, the built-incomponents have the form of plates or discs which can be inserted intothe evaporator. The size and shape of the plates and discs are adjustedto the size and shape of the evaporator. Plates or discs that can beplaced on intermediate bottoms of the evaporator are particularlypreferred. For example, metal lattices or screens are advantageous, e.g.made of expanded metal.

The evaporator according to one embodiment of the invention allowsproviding high-purity vaporous alkyl polysiloxanes which can be easilyprocessed to obtain SiO₂ bodies. After having been dried and thermallytreated, the SiO₂ bodies result in faultless glass blanks which areparticularly suitable for manufacturing glass fibres. By means of themethod according to one embodiment of the invention, the amount ofdeposits in the evaporator is minimised and, at the same time, thepurity of the vaporous alkyl polysiloxanes is maximised.

Preferably, the spraying of the starting materials into the evaporatoris controlled such that a large part of the liquid evaporates withoutcoming into contact with the walls of the evaporation chamber.Preferably, the alkyl polysiloxanes are sprayed through single-substancenozzles or, more preferably, two-substance nozzles which are arranged onthe bottom of the evaporation chamber and spray the liquid into the freechamber in a vertical upward direction. Therein, the spray cone isconfigured such that any wall contact of the droplets is minimised. Theliquid to be evaporated is atomized to form droplets that are as smallas possible. Preferably, the mean droplet size (mean volumetric diameterof the droplets) is within a range from 5 pm to 200 pm, more preferablyfrom 10 pm to 150 pm, and most preferably from 20 pm to 100 pm. Theformation of a fine spray mist allows achieving fast evaporation of theliquid and preventing a high temperature load on the liquid startingmaterial. In the flight phase, the droplets take up heat from theevaporator and largely evaporate without any wall contact. The remainingliquid evaporates during contact with the walls of the evaporationchamber.

According to a preferred embodiment, the liquid is sprayed by means of acarrier gas. The presence of the carrier gas causes a reduction in thedew point. When a carrier gas is used, use is, preferably, made of atwo-substance nozzle such as it is schematically shown in FIGS. 1 and 3.

According to one embodiment of the invention, use is, preferably, madeof pneumatic atomizing nozzles. When pneumatic atomizing nozzles areused, the supply of gas serves to additionally tear up the liquid flowto form ultra-fine droplets. Use can be made of injector nozzles or,preferably, of pressure mixing nozzles.

When injector nozzles are used, the liquid and the gas are mixed by onemedium acting as blowing medium and the second medium aspirating intothe mixing chamber (according to the Venturi principle). Both medialeave the nozzle as a mixture.

When pressure mixing nozzles are used, both media, i.e. gas and liquid,are supplied to the nozzle in a pressurised manner (liquid pressureprinciple).

When pressure mixing nozzles are used, the media can be mixed eitherwithin or without the nozzle body. In case of internally mixing nozzles,both media are mixed in a mixing chamber. The mixture leaves the nozzleopening, e.g., as a hollow cone. In case of externally mixing nozzles,the two media do not meet before they have exited from the nozzle.Mixing is caused by spraying the two spray cones into each other.

Externally mixing nozzles are particularly adapted to atomize viscousmedia tending to contaminate and are, therefore, preferred. Moreover,externally mixing nozzles are to advantage in that the media supplies donot mutually affect each other. Preferably, use is made of an externallymixing two-substance nozzle which comprises a plurality of dischargeopenings that are arranged in the form of a ring.

In general, nozzles with a cone-shaped form of spraying are preferred,more particularly with a full-cone form of spraying.

In order to improve the stirring and spraying of sump residues, one ormore further nozzles 21 can be provided (FIG. 4). For example, theatomizing nozzle 3 can be surrounded by a further annular nozzle or arim of nozzles which are used to introduce the carrier gas. Preferably,this nozzle or these nozzles is/are arranged in an annular recess. Sincemobile residues, preferably, accumulate in recesses, it is, in thismanner, possible to specifically stir up and spray sump residues. Theannular arrangement of the discharge openings is to advantage in thatthe sump residues are carried along and sprayed by the external gas flowwithout impairing the spraying of the starting material through theinternal nozzles.

When a plurality of nozzles or multiple-substance nozzles are used, useis, preferably, made of at least 20% and, more preferably, of at least50% of the carrier gas introduced into the evaporator for atomizing theliquid. The remaining carrier gas can be used for stirring up the sumpmaterial.

In order to accelerate evaporation, the feed material is, preferably,preheated. The temperature depends on the feed material and, in case ofOMCTS, is preferably within a range from 60° C. to 175° C., morepreferably from 100° C. to 160° C.

Preferably, the carrier gas is heated up to a temperature from 100° C.to 250° C., more preferably from 130° C. to 240° C., and most preferablyfrom 150° C. to 220° C.

Preferably, the evaporator itself is also heated. This can be achievedby means of heater elements in the walls and/or bottoms of theevaporator or by means of heater elements inserted into the evaporatorchamber. For heating purposes, use can be made of heated fluids, e.g.

heated liquids or, preferably, electric heater elements. The heating is,preferably, above all and, most preferably, exclusively achieved via thebottoms, i.e. the bottom plate 2 d, the cover 2 e and/or the optionalintermediate bottoms, wherein electrically heated bottoms areparticularly suitable.

Preferably, at least 80% of the heater output are introduced via thebottoms. Preferably, no more than 20% of the heater output areintroduced via the lateral surface wherein, preferably, the lateralsurface should not be additionally heated.

FIG. 2 shows a preferred embodiment of the evaporator according to oneembodiment of the invention which comprises three heatable intermediatebottoms 2 a, 2 b and 2 c in addition to the bottom plate 2 d and thecover 2 e. Preferably, the evaporator comprises at least one and, morepreferably, 2 to 10 and, most preferably, 3 to 6 heatable intermediatebottoms. The intermediate bottoms are each provided with heater elements9 a, 9 b and 9 c via which the bottoms can be electrically heated. Thespray mist exiting from the nozzle 3 is sprayed through the centralopening of the first intermediate bottom 2 a and into the open internalspace 2 f.

The intermediate bottoms 2 a, 2 b and 2 c comprise one or more openingsin order to ensure that the vapour can be discharged in an upwarddirection and condensate and other deposits can be drained in a downwarddirection. FIG. 3 is a top view of the intermediate bottom 2 a. Inaddition to the central opening 11, the bottom has additional holes 12through which the discharging of deposits is facilitated.

The bottom of the evaporator can be designed in various manners. Forexample, the evaporator can comprise a bottom plate with a conicallylowered inner surface or a flat inner surface.

FIG. 4 is an enlarged view of the bottom area of an evaporator accordingto one embodiment of the invention having a flat inner bottom surface.In this embodiment, the bottom plate 2 d also contains at least oneheater element 9 d. The injection nozzle 3 is a two-substance nozzle.

The alkyl polysiloxane is supplied to the nozzle via the inlet 3 a andthe carrier gas via the inlet 3 b. In addition, a further nozzle 21 isprovided via which the carrier gas can be supplied into the evaporator.The carrier gas enters the evaporator via opening(s) 22 which arearranged in the form of a ring around the outlet of the two-substancenozzle 3.

FIG. 5 shows a further embodiment of the bottom plate 2 d with a heaterelement 9 d of an evaporator according to one embodiment of theinvention. The bottom plate comprises a cone-shaped cavity through whichcondensate and gel-like products are collected. They flow to theinjection nozzle 3 where they are carried along by the incoming gasflow. The nozzle 21 can be used to introduce additional carrier gas inorder to stir up the sump products.

The invention claimed is:
 1. A method of manufacturing quartz glasscomprising: evaporating an appropriate liquid starting material capableof being vaporized and oxidized to form SiO₂ by spraying it into avertically arranged evaporation chamber; oxidizing the evaporatedappropriate liquid starting material to form SiO₂; and collecting theSiO₂; characterized in that the appropriate liquid starting material tobe evaporated is sprayed in at a bottom of the evaporation chamber andthe evaporated appropriate liquid starting material is removed at thetop end of the evaporation chamber, wherein the evaporation chamber isdesigned such that components depositing in the evaporation chamberaccumulate on the bottom of the evaporation chamber and are repeatedlysprayed again by an inflowing gas flow in the form of droplets so thatthe vaporizable contents of the deposited components are evaporated andcomponents that do not evaporate polymerize to form a high-viscositypolymer layer having a thickness of less than 70 μm on walls and thebottoms of the evaporation chamber and wherein the components that donot evaporate do not form a sump.
 2. The method of claim 1, wherein thecollected SiO₂is dried; and subjected to a heat treatment to form aglass blank.
 3. The method of claim 1, wherein a polymethylcyclosiloxane is used as an appropriate liquid starting material.
 4. Themethod of claim 3, wherein one of hexamethyl cyclotrisiloxane,octamethyl cyclotetrasiloxane, decamethyl cyclopentasiloxane anddodecamethyl cyclohexasiloxane is used as polymethyl cyclosiloxane. 5.The method of claim 1, wherein the temperature in the evaporationchamber is within a range from 130° C. to 230° C.
 6. The method of claim1, wherein the appropriate liquid starting material is introduced intothe evaporation chamber along with a carrier gas.
 7. The method of claim6, wherein an inert gas is used as the carrier gas.
 8. The method ofclaim 6, wherein the molar ratio of the appropriate liquid startingmaterial to carrier gas is within a range from 0.01 to
 2. 9. The methodof claim 1, wherein the evaporated appropriate liquid starting materialis mixed with oxygen.
 10. The method of claim 9, wherein the evaporatedappropriate liquid starting material mixed with oxygen is introducedinto a burner where it is burned, forming SiO₂.
 11. The method of claim10, wherein an appropriate combustion fuel is additionally introducedinto the burner.
 12. The method of claim 11, wherein hydrogen is used asthe appropriate combustion fuel.
 13. The method of claim 6, wherein theappropriate liquid starting material and the carrier gas are introducedinto the evaporation chamber through a two-substance nozzle.
 14. Themethod of claim 13, wherein the two-substance nozzle is a pressuremixing nozzle.
 15. The method of claim 14, wherein the pressure mixingnozzle is an externally mixing nozzle.
 16. The method of claims 13,wherein the two-substance nozzle comprises discharge openings that arearranged in the form of a ring.
 17. The method of claim 16, wherein thedischarge openings of the two-substance nozzle are surrounded by afurther annular nozzle or a rim of nozzles which are used to introducethe carrier gas.